Generally, commercial electricity supplied by power supply service providers is alternating current electricity. Alternating current electricity is used for driving a load having various electric characteristics.
Recently, other than commercial electricity, private electricity generators have been used as private power sources. Also, electricity sales have been started. In electricity sales, one supplies electricity generated by a private power source to existing commercial electric power systems, and charges for the supplied electricity.
When practicing electricity sales, there is a problem that an inflow of a power flow occurs from the private electricity generator to the commercial electric power system (i.e., a reverse power flow occurs). A reverse power flow causes unfavorable influence on devices driven by the commercial electricity. Further, an integrating wattmeter may not function properly due to a reverse power flow. If the integrating wattmeter malfunctions, the electricity seller cannot receive correct charges for the electricity.
As a method of preventing a reverse power flow, there has been a method of slightly changing the frequency of an alternating current voltage generated by a private electricity generator, monitoring changes of an alternating current, and determining the existence of a reverse power flow in accordance with the monitoring result.
However, according to this method, the structure of a private electricity generator becomes complicated. Further, according to this method, a load to be driven by commercial electricity will be supplied with an alternating current voltage having a complicated waveform. Because of this, electricity cannot be supplied to this load efficiently (that is, the power factor falls). Furthermore, according to this method, other unfavorable influences may be caused on operations of the load.